The role of pH in the melanin biosynthesis pathway.
Having oxidized 3,4-dihydroxyphenylalanine (dopa) with sodium periodate or mushroom tyrosinase in a pH range from 3.5 to 6.0, it has been possible to detect spectrophotometrically 4-(2-carboxy-2-aminoethyl)-1,2-benzoquinone with the amino group protonated (o-dopaquinone-H+), a postulated intermediate in the melanogenesis pathway. When the pH value was greater than 4, the final product obtained was 2-carboxy-2,3-dihydroindole-5,6-quinone (dopachrome); however, for pH values lower than 4, two different products were identified by means of cyclic voltammetry: 5-(2-carboxy-2-aminoethyl)-2-hydroxy-1,4-benzoquinone and dopachrome. These products appeared when oxidation was achieved with the enzyme as well as with periodate. This suggests that two chemical pathways can arise from alpha-dopaquinone-H+, whose relative importance is determined by the pH. The steps of these pathways would be dopa leads to o-dopaquinone-H+ leads to o-dopaquinone leads to leukodopachrome leads to dopachrome, for the first one, and dopa leads to o-dopaquinone-H+ leads to 2,4,5-trihydroxyphenylalanine leads to 5-(2-carboxy-2-aminoethyl)-2-hydroxy-1,4-benzoquinone very slowly leads to intermediate compound leads to dopachrome, for the second one. The stoichiometry for the conversion of dopaquinone-H+ into dopachrome for pH values greater than 4 followed equation of 2 o-dopaquinone-H+ leads to dopa + dopachrome. No participation of oxygen was detected in the conversion of leukodopachrome (2,3-dihydro-5,6-dihydroxyindole-2-carboxylate) into dopachrome.
D016227:Benzoquinones; D055598:Chemical Phenomena; D002621:Chemistry; D004295:Dihydroxyphenylalanine / Q000031:analogs & derivatives / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D045563:Indolequinones; D007211:Indoles / Q000378:metabolism; D007700:Kinetics; D008543:Melanins / Q000096:biosynthesis*; D008956:Models, Chemical; D014442:Monophenol Monooxygenase / Q000378:metabolism; D011809:Quinones / Q000378:metabolism; D013053:Spectrophotometry
D016227:Benzoquinones; D045563:Indolequinones; D007211:Indoles; D008543:Melanins; D011809:Quinones; C035156:leukodopachrome; C001123:dopachrome; D004295:Dihydroxyphenylalanine; C035157:dopaquinone; D014442:Monophenol Monooxygenase